And one of the largest countries on Earth, lying between the

Stratigraphy and structure

The Precambrian

This major period of geologic time can be subdivided into the older Archean and the younger Proterozoic eons, the time boundary between them being some 2.5 billion years ago. In Australia the main outcrop of the Archean and older Proterozoic rocks is in the Yilgarn and Pilbara blocks of the southwest and northwest, respectively.



Hamersley Range, Western Australia

Hamersley Range in the Pilbara region of the Australian Shield, in Western Australia.

Richard Woldendorp/Photo Index

In the Yilgarn block the oldest known rocks are sialic crust (i.e., composed of rocks rich in silica and alumina) that developed in the Narryer Gneiss Complex between 4.3 and 3.7 billion years ago. The older end of that time span is provided by detrital zircon grains found in younger metasedimentary rock (metamorphosed sedimentary rock) some 3.3 to 3.7 billion years old: as determined by ion microprobe analysis, the grains are 4.2 to 4.3 billion years old. A zircon grain imbedded in 3.75-billion-year-old metamorphosed sediment from Jack Hills in Western Australia was found to be even older, 4.4 billion years, and it is thus the oldest dated material on Earth. The younger end of 3.7 billion years ago is provided by samarium-neodymium (Sm-Nd) isotopic analyses of anorthosite and gabbro and more extensive granitic rocks. Subsequent to such igneous rocks being formed, siliceous sedimentary rocks were deposited during an interval of subdued relief and extensive sheets of vein quartz pebbles were concentrated on the surface.

The oldest rocks in the Pilbara block to the north make up a granite-greenstone terrane and so differ distinctly from those of the Yilgarn block. They are mostly 3.3 to 3.5 billion years old and comprise basic (alkaline) volcanics associated with horizontal tabular igneous bodies known as sills and layered intrusions, as well as acid volcanics associated with granitic plutons (bodies of deep-seated intrusive igneous rock) and sheets. The association of basic and acid rocks suggests the possibility that older sialic crust melted. Chert within basalt 3.5 billion years old at the North Pole mining centre in Western Australia contains stromatolites (layered deposits formed by the growth of cyanobacteria) and filamentous colonial microfossils that are among the oldest known sets of fossils on Earth. Between 3.05 and 2.9 billion years ago, thick acid and basic volcanics and sedimentary rocks were intruded by large granite plutons and deformed and metamorphosed to establish the internal form of the Pilbara block. Between 2.8 and 2.7 billion years ago, the beveled surface of the Pilbara block was blanketed by basaltic lava. Finally, between 2.55 to 2.4 billion years ago, banded-iron formation, dolomite, shale, and minor acid-volcanic rocks were intruded by sills of porphyry. Iron ores of hematite and goethite have been formed by supergene enrichment of banded-iron formation.

The Yilgarn block became an internally coherent mass only after greenstone and associated granitic terrane had developed from 3.0 to 2.5 billion years ago, and it was then intruded by a swarm of vertical tabular bodies called dikes composed of dolerite. Mafic and ultramafic rocks (those composed primarily of ferromagnesian—dark-coloured—minerals) 2.7 billion years old within the granite-greenstone terrane are the chief host of the epigenetic gold deposits of Western Australia. Slightly older (2.8 billion years) volcanic ultramafic rocks contain deposits of nickel sulfide.

The Pilbara and Yilgarn blocks were joined between 2.0 to 1.8 billion years ago along a belt of deformed continental-margin deposits. Later in the Proterozoic, between 1.6 billion and 650 million years ago, mountain belts resulting from the collision of continental terranes were repeatedly worn down and overlain by sedimentary rocks. That view contrasts with another interpretation that regards most of the western part of Australia as intact since Archean times and considers that most later orogenic activity was ensialic.

The development of the late Proterozoic Adelaidean province, the other Precambrian succession to be described here, was within a sialic basement. The Adelaidean succession crops out in the region of South Australia between Adelaide and the Flinders Ranges and contains an almost complete sedimentary record of the late Proterozoic. The early Adelaidean Callanna and Burra groups are confined to troughs faulted down into basement. A sheet of sedimentary deposits at the base of the Callanna group was cut by faults into rift valleys that filled with basic volcanic rocks and evaporitic sediment and carbonate rock. The succeeding Burra group comprises fluvial sediment followed by shallow marine carbonate.

The late Adelaidean Umberatana and Wilpena groups unconformably succeed older rocks. The Umberatana group contains a rich record of two glaciations: the older Sturtian glaciation is indicated by glaciomarine diamictites deposited on a shallow shelf and at the bottom of newly rifted troughs; the younger Marinoan glaciation is represented by diamictites deposited on the basin floor and sandstone on the shelf. The Wilpena group comprises extensive sheets of interbedded sandstone, siltstone, and shale deposited during two marine transgressions, during the second of which deep canyons were cut and filled. The uppermost part of the Wilpena group, in the latest Proterozoic, contains the celebrated Ediacara assemblage of the oldest well-known animal fossils.

The Precambrian rocks of Australia provide a rich source of economically important minerals, such as the above-mentioned major iron ore deposits of the Pilbara block and the gold and nickel deposits of the Yilgarn block. Other minerals include diamonds from the Argyle diatreme (vertical volcanic conduit filled with breccia) in northern Western Australia. Lead and zinc are found at Broken Hill in western New South Wales, and lead, zinc, and copper occur at Mount Isa in northwestern Queensland and at Olympic Dam in South Australia.